The systems and methods described herein relate to technologies for delivering fuels and, more particularly, to systems and methods for atomizing fuels for allowing delivery of the atomized fuel to a combustion zone.
Technologies for atomizing fluids and materials are important to a number of industries and to a wide range of applications. One particular application is the delivery of combustible fuels to spark-ignition engines. For this application it is desired to minimize the size of the resulting droplets, or to yield vaporization of the fuel. In fuel delivery systems it is understood that reduced droplet size leads to greater combustion efficiency, which, in turn leads to reduced waste and greater environmental performance.
One common technique for atomizing a liquid fuel is to employ an aspirating gas flow to break-up the liquid into droplets. This technique is employed by carburetors, which are still the predominant fuel delivery system used today for small combustion engines. Although these aspirating systems yield acceptable results for small gasoline powered spark ignition engines, the size of the droplets produced during atomization is still relatively large, and less than optimal for many fuels and more demanding applications. Improvements and modifications that would eliminate or reduce the size of drops provided by the aspiration technique have been suggested, such as screening and flow redirection, but these modification also reduce throughput and create waste problems. Moreover, neither of these improvements address an additional problem with the aspirating technique, which is that the aspirating gas, typically air, can dilute the fuel being delivered, reducing the concentration of the fuel delivered and reducing efficiency of subsequent combustion.
In light of these problems, a considerable amount of research has gone into developing techniques that provide greater atomization of liquid fuels, without requiring an aspirating gas. For example, ultrasonic and electrostatic atomization devices have been developed and employed to further reduce drop size and to eliminate, or reduce, the need for an aspirating gas flow. However, the results achieved with such systems have been mixed and such systems often fail to provide the desired high droplet velocities. Vaporization via heating has also been used, but this technique has resulted in distillation and problems such as fractionation, residue buildup, and decomposition.
By far the most successful alternative to the carburetor has been the fuel injector. Fuel injectors, although more complicated and expensive, provide direct, proportional fuel metering capability under electrical control. Automotive engines in the U.S. now use gasoline fuel injection of two primary types: 1) throttle body injection for the whole engine and 2) port fuel injection for each cylinder. The primary advantages of fuel injection are better specific power (lower fuel consumption per unit power generated) and far better integration with the engine control unit; this results in much lower emissions through better control under a variety of environmental and operating conditions. The control unit can then implement ever more complex and efficient control algorithms, integrating with more sensors to determine optimal settings for the simple control (timing and duration) of the fuel injector, thus further improving fuel economy and reducing exhaust emissions. Accordingly, the fuel injector allows the use of computerized control systems to help optimize performance.
However, fuel injectors efficiencies are still limited by the size of the drops that form the injector spray. Increasingly over the last few years, much experimentation and advanced design has been conducted on the mechanical configuration of fuel injectors to improve their spray/atomization characteristics while keeping their electrical controllability, which is their chief advantage. For example, there has been extensive research and experimentation with forced-air assisted atomization, micro-machining of injection plates, use of flow-swirling effects as well as other approaches. Although significant improvements have occurred, several hurdles still remain. For example, the spray/atomization characteristics of today""s fuel injectors is still insufficient to allow for the use of low vapor pressure fuels, also know as heavy fuels, such as kerosene. To date, the new injector designs and processes do not achieve the very fine atomization and subsequent vaporization required for a successful operation as an spark ignition engine with a heavy fuel. The literature does report that one researcher has successfully cold started a small spark-ignition engine with a crude but effective kerosene vaporizing unit, however this technology is not very controllable nor packageable as a compact fuel injector. Nor is it clear that these systems can provide the very lean mixture that will ignite reliably and controllably at the high compressions ratios needed for the efficiencies demanded today. The approach taken by other researchers has been direct injection of heavy fuels into the engine cylinder. It is not clear however, that these attempts at direct injection vaporization of fuel will actually circumvent the problems of cold start and proper running of kerosene-fueled, spark-ignition engine at low ambient temperatures conditions. Moreover, present research in using heavy fuels with spark ignition engines has shown that the existing techniques fail to deliver fuel in a manner that will effectively ignite, causing the build up of liquid fuel in the engine and the eventual fouling of the spark plug.
Accordingly, there is a need in the art for a fuel delivery system that provides improved vaporization capability to allow for the delivery of conventional low vapor pressure liquid fuels and that can be packaged and engineered as both a direct (in cylinder) or indirect (port/throttle body) fuel injector.
It is further desired to provide fuel delivery systems that can produce atomized volumes of fuel with sufficiently small droplet size to overcome the long-standing problems with cold-start when employing low vapor pressure fuels to fire small, lightweight, heavy fuel-fired, spark-ignition engines.
The systems and methods described herein include fuel atomizers that, inter alia, enable liquid fuels to burn like gases. More specifically, the systems described herein provide fuel atomizer/injectors that enable the use of heavy fuels to fire small, light weight, low compression ratio, spark-ignition piston engines that typically burn gasoline. It is understood that the fuel injection systems described herein create a spray of fine droplets from liquid, or liquid-like filets, by moving the fuels toward their supercritical temperature and releasing the fuels into a region of lower pressure on the gas stability field in the phase diagram associated with the fuels. It is understood that this release into an area of low pressure (relative to the supercritical pressure range of the fuel) causes a fine atomization or vaporization of the fuel. Depending upon the application, gasses, such as oxygen or air, can be entrained or fed into the dispersion to facilitate combustion.
More specifically, the systems and methods of the invention include, in one aspect, processes for injecting a fuel into a combustion engine, wherein the processes involve providing a source of fuel at a pressure near or within the supercritical range associated with the fuel, providing a restrictor adapted for carrying the fuel, passing the fuel through the restrictor and raising the temperature of the fuel passing through the restrictor to a temperature near or within the super critical range associated with the fuel, whereby fuel leaving the restrictor is projected as an atomized spray into a combustion engine. In one practice, the fuel provided to the restrictor is substantially at ambient temperature. To raise the temperature of the fuel, the distal end of the restrictor can be heated so that the fuel passing through the distal end is raised to a temperature within the critical range of the fuel. To this end, the step of providing a restrictor can include a step of providing a restrictor that has its distal end coupled to a heater element, whereby operating the heater element raises the temperature of the fuel passing through the distal end of the restrictor.
In a further practice, the temperature of fuel passing through the restrictor can be controlled to allow for controlling a characteristic of the atomized spray that is representative of an average drop size of particles within the atomized spray. This is understood to allow for the selection of the extent of atomization achieved by the fuel delivery system, from a spray of fine drops to a vaporized volume of fuel, optionally being able to select any point of vaporization in between.
In a further practice, the act of providing a source of fuel can include the acts of providing first and second sources of fuel each being at pressures near the respective supercritical range of the respective fuels and passing the first and second fuels through the restrictor. In this practice, the acts of providing first and second sources of fuels can include providing as the first source of fuel, a fuel capable of acting as a starter fuel, and providing as the second source of fuel, a fuel capable of acting as a primary operating fuel. The fuels can be mixed together and the mixed fuel can be passed through the restrictor. Alternatively, the selection between fuels can be sequential, allowing for delivery of a first fuel and then a second, or third. Further practices allow for the gradual transition between different fuels, by for example allowing a gradual transition between a first fuel and a second fuel, with a mix of the fuels occurring during interim delivery. It will be appreciated by those of ordinary skill in the art that fuels can also be mixed that are supercritical states, as it is understood that fluids in supercritical states are highly effective as solvents. Accordingly, the system can provide a solvent delivery system that mixes compatible or incompatible chemicals just prior to injection into the restrictor, such that the time between mixing and atomization is minimized.
In a further practice, the processes can include the operation of preheating fuel that is being passed through the restrictor. To this end, the processes can provide a preheater for heating a selected volume of fuel, and passing the preheated volume of fuel through the restrictor for delivery to the combustion engine. In one embodiment the systems include a waste heat collector so that the processes can collect waste heat generated by the combustion engine for heating fuel being passed through the restrictor.
Further described herein are fuel delivery systems for delivering an atomized spray of fuel to a combustion zone. These fuel delivery systems can include a fuel pump that is coupled in fluid communication with a source of fuel and that is capable of pumping the fuel at a pressure within the supercritical range of the fuel. The systems can further include a restrictor that has an input port in fluid communication with the fuel pump and an output port, and that is adapted for allowing fuel to pass from the input port to the output port and into the combustion zone. The systems can also include a heater that is coupled to the restrictor and that is capable of heating the restrictor for raising the temperature of the fuel to a temperature within the supercritical range of the fuel, whereby fuel passing through the restrictor is heated to a temperature near or within the supercritical range of fuel for forming an atomized spray that is ejected from the output port and into the combustion zone.
The restrictors can be formed of a tube of electrically resistive material and the heater element can include a source of electrical current that is connected to the tube for delivering current there through. A thermal control unit can operate the heater to control the temperature of fuel passing through the restrictor and, as noted above, can allow for the selection of average drop size of drops within the atomized spray. In some embodiments the fuel delivery systems can pass a plurality of fuels through the restrictor, and to that end can include mixers for mixing fuel for a plurality of fuel sources, as well as a set of controllable valves for sequentially controlling the passing of fuels to the restrictor.
In a further embodiment, the systems described herein include a preheater for heating fuel that is to be passed through the restrictor. In this way the restrictor is provided with a source of preheated fuel, thereby reducing the temperature difference between the temperature of the fuel being passed through the restrictor and the temperature to which the fuel will be raised before the fuel can be placed near or within the supercritical temperature range. In one embodiment, the system includes a preheater that has waste heat collection system that collects heat from the combustion chamber for allowing the preheater to heat fuel being passed to the restrictor. Other preheating systems can be employed, including electrical and flame heaters.
In a further aspect, the invention can be understood as methods for forming a combustible mixture. The processes involve providing a restrictor having an input port and an output port and passage therebetween, delivering a fuel to restrictor input port at a pressure substantially at or near the supercritical pressure of the fuel, passing the pressurized fuel through the passage of the restrictor, heating the pressurized fuel passing through the restrictor to a temperature sufficient to cause an atomized spray to eject from an output port, and injecting the atomized spray into an air intake port carrying a stream of air. In one practice of this method, the fuel being passed through the restrictor can be metered for controlling the ratio of fuel and air being carried through the intake port. Further, the temperature of pressurized fuel passing through the restrictor can be controlled to adjust a characteristic of the atomized spray representative of the average drop size of material in the atomized spray. The atomized spray can be provided to a combustion chamber of an engine, the combustion chamber of a burner, or to any other suitable combustion chamber. The fuels provided through the restrictor can include fuels comprising a low volatility liquid fuel, such as fuels comprising kerosene.
Other aspects and embodiments of the invention will be apparent from the following description of certain illustrative embodiments.